[arcticstorm]

There is yet another possibility.

If I recall correctly, the Valar longed for the day when the children of Illuvitar, elves and men, would live in Arda, but as can be seen in the incident with Aule, the Valar themselves got impatient.

Many times when a little child knows that they have a baby sibling on the way they constantly wonder when the child will be born. The Valar may have realized that if they themselves, got impatient, how much more would the elves get impatient for the coming of their brothers, so-to-speak. And, because even the Valar did not know when, they probably realized that the eldar would be constantly wondering and maybe even asking about the when in hopeful expectation and they would not have an answer. The ten thousand year wait could very easily make the elves disbelieve the Valar's promise that men would come, and thus their trust in the Valar could have been shaken in that way.